Fail-safe torque transducer system

Abstract

A fail-safe torque transducer, and automotive power steering system, utilizing a two-stage series coupled torsion bar system. A first stage torsion bar provides tuning of the steering system, and by reducing its diameter relative to the second stage, provides a lower torsional spring rate and higher operational stress level than that of the second stage. The second stage torsion bar provides the stress / strain indicia referenced by a torque-measuring device, and is stressed at a lower level than the first stage and therefore has a higher cycle life than that of the first stage bar. Hence, if stress related failure ever occurs, it will only occur in the first stage torsion bar, thereby preventing any torque from reaching the second stage, thereby causing a zero strain signal and thus rendering the system fail-safe and not subject to instability of vehicle steering if the torsion bar breaks.

Description

FIELD OF THE INVENTION [0001] This invention relates generally to systems for measuring the torque transmitted through a rotary shaft, and in particular, rotating shafts such as found in hydraulic and electric power steering systems in vehicle applications. BACKGROUND AND SUMMARY OF THE INVENTION [0002] It is well known in the torque-sensing art that the deformation, or twist, of a rotary shaft under load can be sensed as a measure of the torque being transmitted through the shaft. In relatively low torque applications where the amount of twist may be too small for accurate measurement, such as in automotive power steering systems, the twist is typically augmented by inserting a torsion bar between two relatively rigid sections of the steering column shaft. The relative rotation of the more rigid sections of the shaft may be mechanically or electrically detected using a variety of techniques. [0003] In a safety-critical system such as vehicle steering, any single-point failure must ...

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